An overview of the composition and application of biomass ash.

An extended overview of the complex phase-mineral and chemical composition and properties of biomass ash (BA) was conducted in Part 1 of the present work. Then, the identified systematic associations, namely (1) Si-Al-Fe-Na-Ti (mostly glass, silicates and oxyhydroxides); (2) Ca-Mg-Mn (commonly carbonates, oxyhydroxides, glass, silicates and some phosphates and sulphates); and (3) K-P-S-Cl (normally phosphates, sulphates, chlorides, glass and some silicates and carbonates); connected with the occurrence, content and origin of elements and phases in the BA system were used for classification of BAs into four types and six sub-types in Part 1. The potential application of BA using the above classification approach is described in the present Part 2. It is demonstrated that such new BA classification has not only fundamental importance, but also has potential applications in prediction of properties and utilisation connected with the innovative and sustainable utilisation of BAs specified in different types and sub-types. The potential advantages and challenges related to utilisation of BA are described. Different aspects connected with BAs such as: (1) bulk utilisation (for soil amendment and fertilisation; production of construction materials, adsorbents, ceramics and other materials; plus synthesis of minerals); (2) recovery of valuable components and their utilisation (char, water-soluble, cenosphere-plerosphere, magnetic and heavy fractions; and elements); and (3) multicomponent utilisation; are described based on the reference investigations, present data and above classification. Subsequently, additional issues related to BAs, namely: (1) technological advantages and challenges (slagging, fouling and corrosion; low ash-fusion temperatures; erosion and abrasion; co-combustion and co-gasification; prediction of phase composition; and others); and (2) some environmental risks and health concerns (air, water, soil and plant contamination; acidity, alkalinity and leaching; volatilisation, retention, capture and immobilisation of hazardous elements and compounds; ash inhalation and disposal); during biomass and BA processing are also discussed. Finally, it is emphasised that the definitive utilisation, technological and environmental advantages and challenges related to BAs associate preferentially with specific BA types and sub-types and they could be predictable to some extent by using the above combined chemical and phase-mineral classification approaches.